1. Field of the Invention
The present invention relates to methods for determining the amounts of corrosion inhibitor and lubricant to be used in hard disk drives from measurements of the vapor pressure of such constituents. More specifically, the invention relates to a method for measuring vapor pressures of such compounds using a vacuum thermogravimetric analyzer (VTGA), and particularly to a method of accurately calibrating the temperature of the VTGA based on a set of novel Curie temperature standards.
2 Description of the Background Art
Disk drives using magnetic recording of digital information store most of the data in contemporary computer systems. A disk drive has at least one rotating disk with discrete concentric tracks of data Each disk drive also has at least one recording head typically having separate write element and read element for writing and reading the data on the tracks.
As areal densities increase, the impetus to reduce the spacing between the active elements of the head and the disk becomes stronger. Since it is becoming increasingly difficult to reduce the fly height which is a substantial portion of this spacing, it becomes desirable to make other structures that contribute to this overall spacing thinner, such as the protective overcoat layers on the head and the disk. However, as these overcoat layers become thinner, the head and the disk become more susceptible to pinhole formation in these very thin films, ˜<7 nm. Such pinholes act as conduits for attack by corrosive constituents, and by wear in the head/disk enclosure. To overcome the deleterious effects of corrosion and wear of head and disk components having thinner protective overcoat layers, remedies such as corrosion inhibitors and lubricants that are transported in the vapor phase from reservoirs within the head/disk enclosure to the sites of corrosive attack and wear are being developed. However, the cost of these additional chemical remedies can be quite significant. Also, the disk drive must have sufficient amounts of these constituents present during the expected operational lifetime of the disk drive. These two factors make the determination of accurate amounts of such constituents for the charging of internal disk drive reservoirs crucial for cost competitive disk drive technology. Therefore, a critical aspect of such determinations is having accurate data on the vapor pressure of these corrosion inhibitors and lubricants under disk drive operating conditions. A thermogravimetric analyzer (TGA) can be modified to make measurements of vapor pressure. To make vapor pressure measurements, the TGA is modified to make measurements in vacuum of weight loss from compounds of interest. The small changes of weight are measured in vacuum as vapor effuses from a Knudsen cell containing the material of interest, viz. rust inhibitor, or lubricant. When this vacuum TGA, or VTGA, is used to measure vapor pressure, it is found that the expected vapor pressures of known samples deviate significantly from published values. This is probably the result of inefficient heat transfer within the test cell, sample holder, under vacuum, heat being transferred to the sample by infrared radiation, rather than more efficiently by convection or by diffusion at ambient atmospheric pressure, as is the case for non-vacuum TGAs. A variety of methods have been used in attempts to calibrate the VTGA: placing thermocouples in close proximity to the sample, welding thermocouples to the sample holder, and using calibration standards consisting of liquids with known vapor pressures. The method of placing the thermocouple in close proximity to the sample holder is inadequate, because it is difficult to reproducibly locate the thermocouple at the same position from one measurement to the next. The method of welding a thermocouple to the sample holder is less than satisfactory, because it is likewise difficult to produce the tiny weld required. The method of using liquids with known vapor pressures, while in theory is promising, in practice proves to be illusive, because small amounts of impurities significantly alter vapor pressure, and it is difficult to find or produce liquids with sufficient purity for accurate vapor pressure measurements in the temperature ranges of interest.
These problems are exacerbated because vapor pressure is a sensitive function of temperature. For accurate measurements of vapor pressure it is necessary to accurately know the temperature over which the measurement is made. Because of the problems with heat transport in vacuum associated with the VTGA, numerous temperature calibration standards are required to span the limited temperature range of interest. Moreover, the choice of a suitable calibration standard is limited. Typically a standard used for TGA calibration based on magnetic transitions at a Curie temperature is not suitable for the lower temperatures of interest for vapor pressure measurements. Because a typical standard for TGA calibration is fabricated from a pure element to avoid problems with impurities that affect the transition, elemental standards have been the standards of choice and are, therefore, limited to Ni, Fe, and Co, all of which have relatively high Curie temperatures for their magnetic transitions which are unsuitable for vapor pressure measurements at low temperature.
Accordingly one method for calibration at low temperatures suitable for vapor pressure measurements is based on the use of compounds with known vapor pressure. Obtaining vapor pressure standards is often difficult and time consuming due to the elaborate distillation procedures employed to produce vapor pressure standards of sufficient purity.
A calibration standard selected from an alloy or element with a well-known magnetic transition temperature, Curie temperatures, or Curie points, as mentioned above, also have shortcomings. Although a calibration method based on a magnetic standard is desirable, obtaining such standards is difficult. Nevertheless, the relative simplicity of a calibration method based on magnetic standards makes their use appealing. The calibration method using magnetic standards is based on the principle that a magnetic standard, when placed in an external magnetic field, produces a change in apparent weight as the standard undergoes a magnetic transition at its Curie temperature. Specifically, a known calibration method based on a magnetic standard includes: placing a magnetic standard on the balance pan, sample holder, of the TGA; placing a magnet near the standard so that a magnetic force of attraction is exerted on the standard which alters the apparent weight registered on a microbalance to which the pan is attached; as the temperature in the sample chamber is increased, the standard passes through a magnetic transition, becoming non-ferromagnetic upon heating above the Curie temperature, Tc; at the same time, the microbalance registers an effective change in weight associated with the loss of the magnetic force that was previously acting upon the standard below the Curie temperature.
Monel, a CuNi alloy with about 28 to 30% by weight of Cu, has been used as a single standard. This standard is of particular interest because it has a magnetic transition in the low temperature regime at about 65 C. A problem with a calibration method using the Monel standard is that it is virtually the sole standard available in the low temperature regime. However, in attempting to use an available slug of Monel to calibrate a VTGA, a problem with the stability of the standard is encountered. With the passage of time, the Monel standard loses magnetic moment and it no longer exhibits a magnetic transition.
What is needed is a method for accurately calibrating a vacuum thermogravimetric analyzer (VTGA). What is needed is a set of novel standards that free the calibration method from concerns about: placement of thermocouples in proximity to the sample, welding of thermocouples on the sample holder, or purity of vapor pressure standards. What is needed is a set of standards that permits accurate calibration through sufficiently numerous calibration points over a limited low-temperature range for determining vapor pressures of compounds of interest.